A number of hydroformylation processes involve the further processing of vent streams from hydroformylation reactors. The vent streams are present to prevent the build up of inert impurities, such as N2, CO2, Ar, CH4 and hydrocarbons, by purging them from the process. The inerts may get into the process as impurities in the feeds. These are generally vented prior to the product-catalyst separation zone to reduce the load on these separation systems. A significant amount of these inerts are also dissolved in the crude aldehyde product which must be removed as vents or purges in downstream refining. Unfortunately, the process of venting these inerts also tends to result in the loss of valuable reactants, such as olefin.
A number of prior processes have sought to recover and recycle the olefin contained in these vents. However, such processes typically involved complex, expensive designs and many were limited to higher boiling olefins. For example, U.S. Pat. No. 4,287,369 discloses separating unreacted olefin from aldehyde via distillation and recycling (optionally with redistilling) the olefin back to the reaction zone. This approach relies on being able to condense the olefin at the top of the distillation tower to deliver the olefin as a liquid back to the hydroformylation system (or olefin/paraffin distillation system). To achieve this at the pressures contemplated by the '369 patent, the crude aldehyde must be distilled at over 190° C. which will result in high heavies formation. Table 1 of the '369 patent shows that a ΔT of over 100° C. between lines 44 (olefin/paraffin) and 46 (aldehyde) is needed in the distillation column to separate the olefin/paraffin mixture from the aldehyde.
There remains a need for improved hydroformylation processes that are more compact with lower capital requirements. There also remains a need for improved hydroformylation processes that are capable of maintaining high C2-C4 olefin conversion but at lower capital cost.